1. Technical Field
The present disclosure relates to an integrated acoustic transducer in MEMS technology and to the corresponding manufacturing process, and in particular to a microelectromechanical (MEMS) microphone of a capacitive type.
2. Description of the Related Art
As is known, an acoustic transducer, for example, a MEMS microphone, of a capacitive type generally comprises a mobile electrode, in the form of a diaphragm or membrane, arranged facing a fixed electrode, to provide the plates of a capacitor. The mobile electrode is generally anchored, by means of a perimetral portion thereof, to a substrate, whilst a central portion thereof is free to move or bend in response to a pressure of sound wave acting on a surface of the mobile electrode. Since the mobile electrode and the fixed electrode form a capacitor, bending of the membrane that constitutes the mobile electrode causes a variation of capacitance of the capacitor. In use, said variation of capacitance is converted into an electrical signal, supplied as an output signal of the microphone.
As an alternative to MEMS microphones of a capacitive type, there are known MEMS microphones in which the movement of the membrane is detected by means of elements of a piezoresistive, piezoelectric, or optical type, or also exploiting the tunnel effect.
MEMS microphones of a known type are, however, subject to problems deriving from residual stresses (compressive or tensile) within the layer that forms the membrane. The factors that affect stress are multiple, and are due, for example, to the properties of the materials used, to the techniques of deposition of said materials, to the conditions (temperature, pressure, etc.) at which deposition is made, and to possible subsequent thermal treatments.
Residual stresses are frequently the cause of mechanical deformations of the membrane, such as for example warping or buckling, and can significantly affect the performance of the MEMS microphone, for example, reducing the sensitivity thereof.
Even though it is possible to control the amount of residual stress in the membrane by means of an appropriate design of the membrane itself and by evaluating the optimal manufacturing conditions, the result obtained is not satisfactory for applications in which a high sensitivity is required. In these cases in fact, the mechanical behavior in response to stresses of sound waves is in any case dominated by the level of residual stress in the membrane.
In order to overcome these problems, described in the document No. WO 2008/103672 is a MEMS microphone of a capacitive type in which the mobile electrode is suspended over a cavity by means of a single anchorage element fixed with respect to a supporting beam provided in the same layer in which the fixed electrode is formed. The point of coupling of the anchorage element with the mobile electrode is located in the center of the membrane that forms the mobile electrode. In this way, the mobile electrode can release the residual stresses through free radial contractions or expansions.
However, this solution is valid only in the cases in which the residual stresses in the supporting beam are small. If, instead, the supporting beam is subjected to tensile or compressive stresses, it tends to warp in an unforeseeable way, causing a deformation or an inclination of the mobile electrode, which hence assumes a position not parallel to the fixed electrode.
Furthermore, a membrane anchored at the center is very sensitive to the deformations due to the stress gradient.
There can hence occur problems of reduced sensitivity of the microphone during use, and, in more serious cases, a direct contact between the mobile electrode and the fixed electrode.